For example, sounds of a plurality of languages (for example, Japanese and English) are prepared in some videos for content of movies, news, live sports, and the like, and in this case, the reproduction timing of the plurality of sounds is synchronized.
Hereinafter, it is assumed that the sounds with synchronized reproduction timing are each prepared as audio encoded bit streams, and an encoding process, such as AAC (Advanced Audio Coding) including at least MDCT (Modified Discrete Cosine Transform) processing, is executed to apply variable-length coding to the audio encoded bit streams. Note that an MPEG-2 AAC sound encoding system including the MDCT processing is adopted in digital terrestrial television broadcasting (for example, see NPL 1).
FIG. 1 simply illustrates an example of a conventional configuration of an encoding apparatus that applies an encoding process to source data of sound and a decoding apparatus that applies a decoding process to an audio encoded bit stream output from the encoding apparatus.
An encoding apparatus 10 includes an MDCT unit 11, a quantization unit 12, and a variable-length coding unit 13.
The MDCT unit 11 divides source data of sound input from an earlier stage into frames with a predetermined time width and executes MDCT processing such that the previous and next frames overlap with each other. In this way, the MDCT unit 11 converts the source data with values of time domain into values of frequency domain and outputs the values to the quantization unit 12. The quantization unit 12 quantizes the input from the MDCT unit 11 and outputs the values to the variable-length coding unit 13. The variable-length coding unit 13 applies variable-length coding to the quantized values to generate and output an audio encoded bit stream.
A decoding apparatus 20 is mounted on, for example, a reception apparatus that receives broadcasted or distributed content or on a reproduction apparatus that reproduces content recorded in a recording medium, and the decoding apparatus 20 includes a decoding unit 21, an inverse quantization unit 22, and an IMDCT (Inverse MDCT) unit 23.
The decoding unit 21 corresponding to the variable-length coding unit 13 applies a decoding process to the audio encoded bit stream on the basis of frames and outputs a decoding result to the inverse quantization unit 22. The inverse quantization unit 22 corresponding to the quantization unit 12 applies inverse quantization to the decoding result and outputs a processing result to the IMDCT unit 23. The IMDCT unit 23 corresponding to the MDCT unit 11 applies IMDCT processing to the inverse quantization result to reconstruct PCM data corresponding to the source data before encoding. The IMDCT processing by the IMDCT unit 23 will be described in detail.
FIG. 2 illustrates the IMDCT processing by the IMDCT unit 23.
As depicted in FIG. 2, the IMDCT unit 23 applies the IMDCT processing to audio encoded bit streams (inverse quantization results of the audio encoded bit streams) BS1-1 and BS1-2 of two previous and next frames (Frame #1 and Frame #2) to obtain IMDCT-OUT #1-1 as a reverse conversion result. The IMDCT unit 23 also applies the IMDCT processing to audio encoded bit streams (inverse quantization results of the audio encoded bit streams) BS1-2 and BS1-3 of two frames (Frame #2 and Frame #3) overlapping with the audio encoded bit streams described above to obtain IMDCT-OUT #1-2 as a reverse conversion result. The IMDCT unit 23 further applies overlap-and-add to IMDCT-OUT #1-1 and IMDCT-OUT #1-2 to completely reconstruct PCM1-2 that is PCM data corresponding to Frame #2.
PCM data 1-3, . . . corresponding to Frame #3 and later frames are also completely reconstructed by a similar method.
However, the term “completely” used here denotes that the PCM data is reconstructed including the process up to the overlap-and-add, and the term does not denote that the source data is reproduced 100%.